Add support for caching in JwtUtils.decode

[tlbdk]

Add examples with:

• https://github.com/adzerk/node-lru-native
• https://github.com/isaacs/node-lru-cache

class TokenCache {
   constructor(pubKeys) {
      this.pubkeys = {}
      this.tokens = {}
   }
   set(jwt, jwtHeader, jwtBody) {
      this.tokens[jwt] = {
        body: jwtBody,
        expiresAt: Math.max(jwtBody.expiresAt, Date.now() + 3600 * 1000) // Don't cache for more than an hour
     }
   }
   get(jwt) {
     // Check expiresAt
     return this.tokens[jwt].body
   }
}

let tokenCache = new TokenCache()
let decodedJwtBody = JwtUtils.decode(jwt, pubKeys, allowedAudinces, { cache: tokenCache })